Moths Use Sonar-Jamming Defense to Fend Off Hunting Bats

An insect with paper-thin wings may carry much the same defense technology as some of the military's heavy-duty warships.

The finding that a species of tiger moth can jam the sonar of echolocating bats to avoid being eaten seems to be the "first conclusive evidence of sonar jamming in nature," says Aaron Corcoran, a biology PhD student at Wake Forest University and the lead author of the paper reporting the discovery in today's issue of Science. "It demonstrates a new level of escalation in the bat–moth evolutionary arms race."

Before Corcoran's study, scientists were puzzled why certain species of tiger moths made sound. Some speculated that the moths use it to startle bats. A few pointed to its potential interference with their echolocation. General consensus, however, fell with a third hypothesis: clicks function to warn a predator not to eat the clicking prey because it is toxic, or at least pretending to be.

To test these hypotheses, Corcoran and his team pitted the tiger moth Bertholdia trigona against the big brown bat Eptesicus fuscus, a battle frequently fought after sundown from Central America to Colorado. High-speed infrared cameras and an ultrasonic microphone recorded the action over nine consecutive nights.

The process of elimination began. If moth clicks served to startle, previous studies suggested the bats should become tolerant of the sound within two or three days. "But that's not what we found," says Corcoran, explaining the lack of success bats had in capturing their clicking prey even through the last nights of the study.

How about the toxic warning theory? If this were the case, according to Corcoran, bats would not find the moths palatable or, if they were indeed tasty, they would quickly learn they'd been tricked. Either way, bats should start to ignore the moth's unique ultrasonic clicks. Also, bats partook readily when offered B. trigona that lacked the ability to click, and they kept coming back for more. This attraction also held true for clicking B. trigona: The predators persisted after their prey despite only reaching them about 20 percent of the time. Bats actually launched four times as many successful attacks against a control group of silent moths.

These findings are "only consistent with the jamming hypothesis," Corcoran notes. "But the most distinctive evidence was in the echolocation sequences of the bats."

Normally, a bat attack starts with relatively intermittent sounds. They then increase in frequency—up to 200 cries per second—as the bat gets closer to the moth "so it knows where the moth is at that critical moment," Corcoran explains. But his research showed that just as bats were increasing their click frequency, moths "turn on sound production full blast," clicking at a rate of up to 4,500 times a second. This furious clicking by the moths reversed the bats' pattern—the frequency of bat sonar decreased, rather than increased, as it approached its prey, suggesting that it lost its target.

The biological mechanism behind the moth's defense strategy is still unclear to researchers. "Most likely, moth clicks are disrupting the bat's neural processing of when echoes return," Corcoran says. Bats judge how far away a moth is based on the time delay between making the cry and its audible return. This "blurring" of the bat's vision, he explains, "may be just enough to keep the moth safe."